Also known as Range
Full scale is the nominal maximum flow rate that a particular thermal
mass flow meter or mass flow controller will measure and/or control. In the
example of an analog flow instrument with 0-5 VDC output, the range is the flow
rate at which the instrument will give an output of 5 VDC.
Overrange state occurs when the flow rate exceeds the range or full scale of the flow
instrument. Many mass flow instruments can handle flow rates that are slightly
overrange. Some models can even measure to 120% and beyond. However, the
accuracy of the flow instrument in the overrange state will very likely deteriorate
as the range of the model is exceeded.
Turndown Ratio defines the usable range that a mass flow meter or controller can be used while maintaining its published accuracy:
Turndown Ratio = Full Scale Flow / Minimum Flow
Most analog mass flow meters have an accuracy of ± 1% of Full Scale (FS) and have resolution better than 1%. The usable range is from 1% to 100%. They will have a turndown ratio of 100/1 or more commonly expressed as 100:1. Digital flow meters will have an even greater turndown ratio due to their higher.
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|Accuracy as % of Full Scale||Accuracy as % of full scale is the difference between the indicated flow rate of an instrument under test (IUT) when compared to the actual flow rate as measured by an accepted flow standard (Std) expressed as percent of the full scale:|
% Full Scale = (IUT – Std) / Full Scale of IUT
For example, if a 100 SLM flow instrument is specified to be ±1% full scale, then the uncertainty is ±1 SLM across the entire range (0-100 SLM).
|Accuracy as % of Reading|
called Accuracy as % of Point. It is the difference between the indicated flow
rate of an instrument under test (IUT) when compared to the actual flow rate as
measured by an accepted flow standard (Std) expressed as percent of the actual
% Reading = (IUT - Std) / Std
in measurement is a value that describes the spread in measurement values that
is related to the measurement technique itself. Standard deviation is often
used to characterize uncertainty.
Hysteresis is the
difference in the accuracy of a measurement when approached from above the
setpoint and the accuracy of a measurement when approached from below the
|NIST Traceable Calibration|
NIST traceable calibration is using metrology instrumentation that has a calibration trail leading back to
standards residing at NIST (National Institute of Standards & Technology).
Note that a NIST traceable calibration does not necessarily mean that the
calibration has a low-level of uncertainty. An optional NIST documentation package
includes all documents (i.e. the paper trail) linking the calibration back to
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|Linearity||Linearity is the difference the ideal straight line from zero flow to full scale is compared to the actual flow indication of the instrument under test. Linearity is often expressed as a percentage of full scale.|
Repeatability is an
instrument’s ability to produce the same outcome given the same operating conditions
using the same instrument. It is often expressed as a % of full scale.
Reproducibility is the
closeness of agreement between the measurements of a value among different
instruments, usually over longer time periods with typical variation in
environmental conditions. A measurement is made with multiple instruments at a
setpoint. The standard deviation of the values obtained is the reproducibility.
It is usually specified by the worst case setpoint as a 3σ value.
a flow instrument, a totalizer sums the amount of gas that has passed through
the instrument over a period of time. It may be a separate device or an
internal register within the flow instrument. This is convenient for
applications that require the recording of total gas use, not only the flow
|Gas Correction Factor (GCF)|
GCF is a multiplier that allows the user to convert the calibration of a
thermal mass flow instrument from one gas to another. The GCF will vary by gas
due to the differences in density and specific heat.
|Nitrogen Equivalent Flow Rate|
Nitrogen equivalent flow rate is a number used to describe the nominal flow rate that a user would expect if
the flow instrument was used in nitrogen gas. Nitrogen equivalent flow, when
expressed in sccm or slm, will typically have reference conditions of 0°C & 760 Torr. The number is very useful for
determining the correct size of the instrument’s body, fittings, and other
Calibration gas is the actual
gas used during the calibration of a mass flow meter or mass flow controller.
However, instruments are often calibrated for use in gases other than the
calibration gas by the use of calculations and conversion of the transfer flow
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Warm Up Time
Warm up time is the time
required for the flow instrument to reach thermal equilibrium such that it performs
within its published specifications, including accuracy and response time.
|Maximum Allowable Working Pressure||The maximum pressure that a device can be exposed to without causing permanent damage to the operation of the unit. |
Working or Operating Pressure
The pressure range over which the device is designed to operate. For pressure devices, this would be the
measurement range. For flow devices, this is the line pressure that the device is intended to operate.
is the pressure at which the device will rupture due to the application of
known as response time. In a mass flow instrument, it is the amount of time
elapsed from the moment the flow command is changed until the flow instrument
reading stabilizes to within an acceptable error band of the desired flow.
Time can be defined in several ways. The first way is by the time constant of
an exponentially rising output after a step change in input. One time constant
(τ) is the time for the output to reach (1-1/e) or approximately 63.2% of its
final value. A second way is the Rise Time, or the time it takes the output to
go from one level to another. Frequently, this is 10% to 90% or 0% to 100%. The
first two methods can be misleading if a system response is not exponential or
exhibits overshoot. A third way is the Settling Time, which is the time
required for the output to reach and remain within a given error band around
its final value following a step change in setpoint. We typically use a ± 2%
error band and a step change from 10% to 100%. Using 10% removes some
variability due to valve preload and pneumatic forces. Other step changes can
be used as well, such as 0% – 100% or 10% to 90%.
Wetted materials are the materials
in the instrument that are exposed (i.e. direct contact) to the gas medium
during the operation.
Metal sealed is a mass flow instrument (meter or controller) which contains no elastomeric seals
with the exception of the valve seat. Metal sealed versions offer higher purity
than elastomeric instruments and fewer wetted material issues when using
Elastomeric sealed refers to the use of o-rings in a mass flow instrument to ensure that the unit
does not leak. Seals are typically made using Viton®, but
other elastomeric material such as Buna-N, Neoprene, and Kalrez® may be available.
Kalrez® is a Perfluoro
elastomer (FFKM/FFPM) manufactured by DuPont™. This elastomer is known for its
resistance to even the most corrosive gases, such as Hydrofluoric acid (HF).
Viton® is a fluoropolymer elastomer, trademarked by DuPont™, and designed to be highly
versatile against various environments and commonly used in o-rings. It is
resistant to most gases and this makes it highly favored in chemical, petroleum,
and aerospace industries.
Buna-N a nitrile rubber that is commonly used today. It is often used in flow
controllers that handle carbon dioxide. It is less expensive than other
elastomers such as Kalrez® and it does not allow the CO2 to absorb into
the o-ring which may cause explosive decompression.
Neoprene is a synthetic rubber that is known for its chemical stability and flexibility
over a wide temperature range.
Oxygen cleaning is a process in which the flow instrument is made safe for use in oxygen by ensuring
that all hydrocarbon and other combustible materials are removed. Typically,
the instrument is thoroughly dissembled into its individual components and then
each is cleaned. During reassembly in a clean area, special lubricants may be
required. Upon completion of cleaning, the flow instrument is bagged to ensure
that it does not become contaminated.
|Monel||Monel is nickel alloy that has excellent resistance to corrosive gases particularly halogens.|